The applicants' long-term Aim is to achieve a deeper understanding of how the brain interacts with the environment to produce coordinated voluntary movement through the control of the neuromuscular apparatus. The present study is based upon the model that they have developed for controlling limb movements. It proceeds from the premise that the ultimate goal of any motor theory is to be able to predict the behavior of measurable variables, not merely in terms of each other, but in terms of the externally given definition of the movement task. The unifying theme of all the proposed experiments is to expand and validate the model as a basis for understanding how the central nervous system performs simple movements and continually adapts to ever changing tasks and environments. Three series of experiments are proposed to study single-joint, flexion-extension movement at the elbow. The first series addresses the question of kinematic/dynamic novelty. It examines how one controls a movement that is done without proper repetitions, i.e. how one makes individual movements, each different from its immediate predecessors (as are done in normal behavior), as opposed to making repetitive and identical movements (as are done in most experimental designs). The second series extends this to study how central commands are adapted to deal with different forms of novelty in the control of the voluntary task which arise from external changes in the environment. The third series examines another form of adaptation that occurs when subjects experience fatigue while performing simple motor tasks. It is indicated by the applicants, to their surprise, although the mechanisms that cause neuromuscular fatigue have been extensively studied, little is known about how the performance of dynamic movements is degraded by fatigue, or how the nervous system attempts to adapt and compensate. In the same way that they intend to characterize the adaptation of central commands to external changes in the task and environment, they will seek to characterize adaptation to internal changes brought about by fatigue.